AC/A Ratio Flashcards
What is accommodative convergence?
Component of convergence that occurs because of accommodation that can be calculated through the AC/A ratio
What is the AC/A ratio?
The amount of accommodative convergence (in PD) that occurs per dioptre of accommodation which is typically a 4:1 (AC/A)
This means for every 1D of accommodation there’s 4PD of accommodative convergence BUT isn’t always 4:1 for everyone
What does the AC/A ratio measure?
Measures the effect of accommodation on the angle of deviation and can be measured by changing accommodation level and assessing the effect on deviation
How do you calculate the AC/A ratio?
Change the stimulus to accommodation and measure the amount of accommodative convergence induced by the change
The change in accommodation level also effects angle of deviation so if accommodation is relaxed in an Exo we can see how the measurements change when we INDUCE accommodation
BUT, in an Eso where accommodation is too much we can see how the measurements change when we relax accommodation
When calculating AC/A what must we be aware of?
Tonic and proximal convergence
What is the Gradient Method?
A method of calculating AC/A ratio - changes the amount of accommodation by using convex and concave lenses. It’s the most accurate method as it excludes both proximal and tonic convergence
What is the Heterophoria Method?
A method of calculating AC/A - change the amount of accommodation by changing the distance of the fixation target
How does the gradient method work with minus (concave) or plus (convex) lenses?
Minus (concave) at 6m (-3D)
Plus (convex) at 1/3m (+3D)
How do you use the gradient method to work out a minus lens at 6m?
(Measurement with induced accommodation - Measurement without accommodation) / Change in accommodation
REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE
How do you use the gradient method to work out a plus lens at 1/3m?
(Measurement on accommodation - measurement with relaxed accommodation) / Change in accommodation
REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE
What’s the gradient method formula for convex and concave lenses?
(Measurement with accommodation - Measurement without accommodation) / Change in accommodation (the lens amount)
REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE
What’s the method for the gradient methods?
- Use trial frames with lenses over either eye
- Perform with glasses if worn
- Use detailed accommodation target
- Allow time for accommodation to take place & ensure the patient can see the target clearly
- Maintain dissociation (occlusion) throughout the PCT
REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE
PCT
6m: 45BI XT
6m c/ -3DS: 30BI XT
What’s the AC/A ratio using the gradient method?
(Measurement on accommodation - Measurement without accommodation) / Change in accommodation
(-30) - (-45) / 3 = -5
AC/A - 5:1
Used negative values because both are EXO deviations
PCT
1/3m: 18BO ET
1/3m c/ +3DS: 3BO E
What’s the AC/A ratio using the gradient method?
(Measurement on accommodation - Measurement with relaxed accommodation) / Change in accommodation
(18) - (3) / 3
AC/A - 5:1
Used positive values because both are ESO’s
PCT
1/3m: 3BO E
1/3m c/ +3DS : 6BI X
Need to be aware that one is an eso (+ sign) & one is exo (- sign) so:
(Measurement on accommodation - Measurement without accommodation) / Changes in accommodation
(+3) - (-6) / 3 = 3
AC/A = 3:1
How do you conduct the gradient method on the Synoptophore?
The patient isn’t accommodating on a synoptophore as it’s a distance test so can only do the 6m formula:
(Measurement on accommodation - Measurement without accommodation) / Change in accommodation
- Test with any refractive correction
- Correct IPD
- Use foveal simultaneous perception slides
- Measure objective angle in PD
- Put -3DS lenses in the lens holders of trial frames
- Ask if the picture is clear as if it’s blurred then decrease lens strength like to reduce to -2DS
- Re-measure objectively
- Use distance test 6m with the minus lens
What’s the formula for the Heterophoria method?
AC/A = IPD + ((n - d) / D)
IPD = Interpupillary Distance
n = PCT results at 33cm
d = PCT results at 6m
D = Accommodation in D which is calculated by 1/focal distance
IPD is added to allow for fixation movement created by change in fixation distance
What’s the method of the Heterophoria method?
- Refractive correction if worn
- PCT measured at near & distance
- Measure IPD
- Measure AC/A using the above
What does the Heterophoria method not eliminate?
Doesn’t eliminate proximal convergence and there’s a change in fixation distance
Why is AC/A important in Eso?
A normal AC/A ratio is 4:1 so for every 1 dioptre of accommodation we have 4PD of accommodative convergence
A high AC/A ratio may be 8:1 which would cause an Eso to get bigger, hypermetropic/convex/+ve will help control it as it relaxes the accommodation
E.g. Convergence Excess ET
1/3m: Near ET when looking at target
6m: Esophoria
= but need more convex at near to relax accommodation but distance so need bifocals
Why is AC/A important in Exo?
If someone has an intermittent XT we want to know if it’s “real” or if due to a high AC/A ratio:
True would have near X and distance XT
A stimulated XT would be both a near XT and distance XT (so due a high AC/A ratio they could be controlling it at near)
What’s the accommodation in dioptres for a 24, 32, 40, 48 & 56 yo?
24 - 11.3
32 - 8.9
40 - 6.5
48 - 4.1
46 - 1.7
(Goes down 2.4 per 8 years)
What is near point of accommodation?
Closest point at which an object can be clearly seen
What is far point of accommodation?
Furthest point at which an object can be seen clearly
